Internal combustion gas-powered hand tools, such as fastener driving tools, are well known in the art. U.S. Pat. No. 4,403,722 to Nikolich and U.S. Pat. No. 5,090,606 to Torii et al., disclose two such tools. Both of these patents disclose portable or self-contained fastener driving tools, i.e., the tools include their own source of fuel (typically propane).
One of the persistent issues in the development of gas-powered tools is reliable ignition of the fuel-air mixture and generation of sufficient power for driving nails or performing other high-power requirement tasks. The flammability limits of propane in air are about 2.2% to 9.5% by volume. When combusted, fuel-to-air ratios in the mid to low end of this range ("lean" mixtures) release the most energy, provide the greatest driving force, and use the fuel most efficiently.
Lean mixtures, however, are often difficult to ignite. Fuel-to-air ratios in the mid to high range ("rich" mixtures) release relatively less energy, produce less driving force, and use more fuel per cycle. Rich mixtures, however, are typically more easily ignited than lean mixtures. The hand tools disclosed in the Torii and Nikolich patents, for example, use a system of baffles or a fan within the combustion chamber to enhance mixing of the fuel-air mixture to provide more reliable and efficient ignition, particularly for lean mixtures.
Although the tools shown in Torii and Nikolich may function generally satisfactorily, the internal construction of the tools is very complicated, employing reciprocating cylinders or sleeves that require o-ring seals and resulting in a serpentine path for introduction of fresh air and/or the exhaust of combustion products. One of the significant drawbacks with the complicated construction is that it adds to the manufacturing and assembly cost, as well as to the weight of the device, which is important for portability.
In addition, the indirect and tortuous flowpath for exhaust and replacement air inhibits the evacuation or "scavenging" of the gaseous combustion products and unburned fuel from the interior of the tool. If uncombusted fuel remains in the combustion chamber it is difficult to accurately control the fuel-to-air mixture in the subsequent combustion cycle, which is required for maximizing the efficiency of the tool. Incomplete scavenging may result in the fuel-to-air ratio in subsequent cycles being higher than desired, leading to less power.
Accordingly, it is an object of the present invention to provide internal combustion gas-powered self-contained tool that has increased efficiency of operation. More particularly, it is an object of the present invention to provide such an internal combustion tool that utilizes an improved scavenging system. It is a further object to provide such a tool that accurately delivers an appropriate amount of fuel to the combustion chamber so that optimal the fuel-to-air ratio can be attained. It is another object of the present invention to provide an internal combustion tool that may be efficiently manufactured and assembled. It is a still further object to provide an improved sparking device or spark source for such a tool so as to provide more reliable combustion of lean fuel-to-air mixtures.